Electrical and electrochemical properties, such as conductivity and dielectric constant, are often used to assess the condition of oil and other fluids. These measurements have traditionally limited the response of the measurement to specific frequencies only and therefore do not consider the overall spectrum response of the system. Additionally, the measurement is typically accomplished using one or more fixed-amplitude, single frequency tones. In most cases, the magnitude of the response is used as the sole gauge. The phase change of the response, which contains information needed to evaluate capacitance and inductance changes, is rarely used in field applications. For example, U.S. Pat. Nos. 4,646,070 (Yasuhara) and 6,028,433 (Cheiky-Zelina) disclose designs in which only one frequency tone is evaluated. U.S. Pat. No. 6,583,631 (Park) presents a method of determining only capacitance. Similarly, U.S. Pat. No. 6,535,001 presents a capacitive sensor that outputs a single DC voltage level, while U.S. Pat. No. 6,459,995 relies on a fixed frequency tone of an LC oscillator circuit to produce the interrogation signal. These designs provide little information about the full electrochemical response of the fluid. Furthermore, these methods neglect useful information that can be extracted from the fluid's broadband impedance. For those systems that do consider a multitude of frequencies, the fluid is repeatedly interrogated by a single frequency waveform, which results in full fluid characterization taking an extended time, up to 50 minutes (as disclosed in U.S. Pat. No. 6,577,112 by Lvovich). This approach is susceptible to very large errors due to environmental changes that can occur during the interrogation window. U.S. Pat. No. 5,889,200 describes a sensor that interrogates a fluid simultaneously using a multitude of frequencies in the form of a square wave. However, only one measurement (conductivity) is extracted and no effort is made to evaluate the fluid's broadband impedance. A square wave is also inferior to the interrogation signal presented by the current invention in the inability to control the signal's amplitude at specific frequencies. A similar design presented in U.S. Pat. No. 5,274,335, employs a triangle wave for interrogation, which suffers the same drawbacks as the square wave interrogation signal.
In most cases, the failure mechanism that dominates a mechanical system can be traced back to the fluid quality degradation or contamination of the system. It is precisely for this reason that on-line, in situ oil quality analysis is the key building block to effective diagnostics and prognostics for mechanical systems. The present invention directly addresses, this need in addition to the aforementioned technology shortcomings, with a novel sensor package to determine a fluid's broadband electrical impedance, which can be used to, among other things, predict quality and degradation in a range of fluid systems.
One aspect of the present invention is a measurement system comprising: a low-powered, broadband, interrogation signal; the analog circuitry needed to condition and facilitate acquisition of the interrogation (and response) signal(s); a data acquisition device for capturing these signals; and a processor and algorithms to control the interrogation and acquisition process as well as interpret the measurements to determine the impedance of the fluid.
In accordance with another aspect of the present invention, there is provided a method for measuring a fluid's impedance as a response to an interrogation, comprising: injecting a broadband signal containing a range of frequencies (range is dependent upon fluid type); and measuring the response to such signals through a fluid to determine impedance.
As part of this invention, a digital to analog converter is used to generate sensor interrogation waveforms comprising a composite of sinusoidal waveforms of varying frequency. A measurement circuit provides an analog to digital converter with inputs corresponding to the original interrogation signal and the sensor's response to that signal. A processor, in the form of a microcontroller, digital signal processor, a remote computer, etc. performs analysis of the response signals using a set of algorithms designed to calculate the impedance of the fluid based on magnitude and phase measurements extracted from the digitized input signals. In one embodiment, the sensor electrodes are constructed of two conductive plates that allow a representative fluid sample to pass between the plate surfaces. There is no intent to restrict the geometry of the electrodes to solely parallel plate designs; concentric rings, coaxial cylinders, and redundant (multiple version of a given design allowing a redundant measurement) electrodes should also be considered.
The measurement produced by this invention can be processed for the purpose of tracking specific electrochemical properties (conductance, capacitance, dielectric constant, inductance, and derived combinations), which have been demonstrated to be an effective method to sense changes in fluid quality, as indicated by Saba, C. S., and Wolf, J. D., “Tandem Technique for Fluid Testing”, Joint Oil Analysis Proceedings, 1998, pp. 81-90; Brown, R. W., et al., Novel Sensors for Portable Oil Analyzers, Joint Oil Analysis Proceedings, 1998, pp. 91-100; and Brown, R. W., and Cheng, Y., “Mathematical Physics Optimization of Electrical Sensors for Contaminant Detection”, 7th Annual Users Conference, Las Vegas, Nev., October 1996. However, there is no intent to limit the invention for use in determining oil quality and the application of impedance measurement to other fluids, liquid plastics, and other 2-phase or variance substance problems is implied.
The present invention will be described in connection with a preferred embodiment, however, it will be understood that there is no intent to limit the invention to the embodiment described. On the contrary, the intent is to cover all alternatives, modifications, and equivalents as may be included within the spirit and scope of the appended claims.